The use of smoke alarms in recirculating air conditioning systems is well known. The smoke alarms are normally mounted near or on the ceilings in various rooms serviced by the recirculating system or mounted adjacent to ducts within the system. When mounted in proximity to a duct, smoke alarms typically require an opening in the duct to sample the air or transmit some form of electromagnetic radiation through the air of the duct to detect smoke. While these types of smoke alarms are effective for generating alarms when smoke is detected in a room or duct, they usually require mountings, installation, and maintenance separate and distinct from the other components in the system.
For example, U.S. Pat. No. 3,369,346 shows a smoke alarm mounted in an auxiliary duct for a fiber carrying airstream. A portion of the fiber carrying airstream is diverted into the auxiliary duct so the smoke alarm can sense smoke in the diverted airstream. The smoke alarm of U.S. Pat. No. 2,474,221 uses reflected light to detect smoke within a duct. The smoke alarm of this '221 patent is mounted directly to the outside of one wall of the duct. An opening in the duct is required so a photoelectric sensor connected to the alarm can extend into the airflow. Light is injected into the duct through the opening by the alarm and the sensor detects reflected light from the particulate in the duct. The apparatus of U.S. Pat. No. 3,885,162 also uses optical techniques to detect smoke but does not include a sensor that extends into the airflow. Rather, a second opening is cut in the duct which opposes the light source of the alarm.
The operational components of the above described devices and other similar devices are mounted to the duct in a manner that facilitates their maintenance and keeps the components in a relatively clean operating environment. Environmental considerations for the electronics are important because temperature fluctuations created by the air flowing through the duct thermally stresses the electrical components and the suspended particulate in the air flow may disable certain types of sensors by blocking the flow of air through the sensor. These and other requirements have placed limitations on the development of devices for smoke detection in domestic and industrial buildings.
There is a continuing need for improvements in a forced recirculating air conditioning system that detects smoke in the air flow promptly and effectively.